Vitamin D has important effects upon calcium and bone metabolism and may be involved in the functioning of many other systems. Vitamin D insufficiency contributes to bone loss in the elderly and has recently been found to be quite prevalent in the northeast US. In both rodents and humans, adipose tissue is a major repository for vitamin D, however the function of adipose tissue in vitamin D metabolism is uncertain and factors affecting the assimilation and release of vitamin D from adipose tissue need to be elucidated. The overall purpose of this research project is to begin to define the role of adipose tissue in vitamin D metabolism. In this regard, we hypothesize that: 1. Vitamin D intake influences the level of vitamin D in adipose tissue 2. The vitamin D content of adipose tissue varies with season 3. Higher fat stores of vitamin D at the end of the summer will reduce the wintertime decline in blood 25-hydroxy vitamin D levels. To test these hypotheses, detailed investigations of vitamin D content in adipose tissue, body composition measurements, blood tests and dietary intake will be conducted in adult men and women participating in a longitudinal study. The proposed study consists of a seasonal observational period followed by a randomized placebo control trial of varying doses of oral vitamin D. In preparation for this study, I will train in vitamin D assay techniques and help refine the method for extracting and measuring vitamin D content in adipose tissue. Studying the role of adipose tissue in vitamin D metabolism will help increase our understanding of vitamin D physiology. It may also enable us to better understand the mechanisms involved in vitamin D insufficiency and give us greater insight into the management of this prevalent condition. In addition, the proposed study sets the groundwork for future studies that are essential to my development as an independent investigator. The proposed study as well as the other components of the K23 Award will enable me to broaden my skills in clinical research and increase my depth of knowledge and technical skills. The supportive environment and distinguished collaborators that are part of this proposal will provide me with the guidance that I need to pursue my research plans as I make the transition to independent clinical investigator.